1. Field of the Invention
The present invention generally relates to a digital signal processor and a modem using the digital signal processor, and more particularly, to a digital signal processor having an external ROM storing a program, forwarding a program to be executed to an internal program RAM if necessary, and executing an instruction stored in the internal program RAM, and a modem using the digital signal processor.
2. Description of the Related Art
Recent progress of digital signal processor (DSP) has expanded the scale of a processable digital signal process. The scale of a program RAM provided in the DSP is limited due to cost problems and the like.
Therefore, for execution of a program that is beyond the capacity of the internal program RAM of a small scale, an external ROM having an area for storing programs is used together with the DSP. The program is read from the external ROM and is transferred to the internal program RAM of the DSP as necessary. For instance, Japanese Laid-Open Patent Application No. 9-31184 discloses a signal processing apparatus using a DSP as described above. The signal processing device includes a plurality of DSPs and a ROM that is provided outside of the DSPs and stores programs. Programs are transferred from the ROM to internal program RAMs of the DSPs at the time of starting up the apparatus. The DSPs execute the programs stored in the internal program RAMs. Another signal processing apparatus is disclosed in Japanese Laid-Open Patent Application No. 2000-40028. The apparatus has external ROM storing programs, a DP-RAM and a CPU, these components being provided outside of a DSP. The CPU forwards a program needed to the DP-RAM from the external ROM. Then, a reset signal that has been applied to the DSP is removed. This results in activating the DSP, which fetches the program from the DP-RAM and stores it in an internal program RAM. Then, the DSP starts to execute the program in the internal program RAM. However, Japanese Laid-Open Patent Application No. 9-31184 and the Japanese Laid-Open Patent Application No. 2000-40028 have the following disadvantage. When the program in the program RAM built in the DSP is replaced, the re-set signal is asserted to the DSP. After the program is replaced by another program, the DSP is initialized. For a DSP having an I/O port, it is therefore necessary to reset the I/O port for each replacement. This degrades the performance of the DSP. In the meantime, recent progress of the DSP has made it possible to implement data modulation/demodulation of a modem by DSP and to improve the transmission rate. Nowadays, an increased number of modulation/demodulation formula is available. However, it is difficult in terms of cost to store all programs respectively describing the modulation/demodulation formula in the internal program RAM in the DSP.
In addition, new recommendations about the modem are added with a short cycle. In order to follow the new recommendations, it is necessary to replace the DSP by a new DSP in which programs corresponding to the new recommendations are described. This is disadvantageous to manufacturers and customers. Taking into account the above, a recent modem is equipped with a DSP having a small-scale program RAM and a rewritable ROM provided outside of the DPS. A program stored in the ROM is transferred to the program RAM in the DSP as necessary. The use of the external ROM easily copes with addition of recommendations by updating. Japanese Laid-Open Patent Application No. 6-164655 discloses a modem which uses a DSP having an external ROM storing a program, forwarding a program to be executed from the ROM to an internal program RAM, and executing instructions stored in the internal program RAM.
However, the program for data modulation/demodulation should be replaced promptly in the modem with the DSP because the replacement is carried out during communication.
Conventionally, a reset signal is asserted to the DSP for replacement of program, and is removed to activate the DSP. Just after activation following the removal of the reset signal, an internal parameter in the DSP and a parameter in an analog front end (AFE) have been initialized by the reset signal. Thus, it is necessary to reset the above parameters. Accordingly, it takes a long time to start the modulation/demodulation program and communication may be broken down in practice.
Furthermore, even for the same data modulation/demodulation formula, parameters with respect to the receiving and transmission are changed based on the change of a carrier frequency and a modulation rate although the algorithm is unchanged. In the above state, it is necessary to change all programs in compliance with the change of the carrier frequency and the modulation rate, because the internal program RAM is initialized by the reset signal and it is impossible to change a part of the program. Otherwise, it is necessary to load every parameter to the program RAM with the same modulation form.
However, there is a problem that it takes a long time to change all programs. There is another program that loading of every parameter needs a large scale of the internal program RAM, which increases the cost.